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Problem:
Implement the following operations of a queue using stacks.
- push(x) – Push element x to the back of queue.
- pop() – Removes the element from in front of queue.
- peek() – Get the front element.
Notes:
- You must use onlystandard operations of a stack – which means only push to top,peek/pop from top, size, and is empty operations are valid.
- Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
- You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
Analysis:
题意:用栈实现队列
用两个堆栈实现,一个放push的数据,另一个放pop的数据,但是在pop在前需要将push栈中的数据push到pop栈中,这样就可以使pop先进先出了。
注意:
- 堆栈的输入元素是对象,所以需要定义Integer类型
- pop时应该先pop堆栈(pop的),如果为空就要将push堆栈的数据push到pop堆栈中,然后再pop出去。
- peek需要返回int类型,所以注意输出时return,不要忘了。
- 判断是否为空主要是判断两个栈是否同时为空。
Anwser:
class MyQueue {// Push element x to the back of queue.Stack<Integer> in = new Stack<>();Stack<Integer> out = new Stack<>();public void push(int x) {in.push(x);}// Removes the element from in front of queue.public void pop() {if (!out.empty()) out.pop();else {while(!in.empty()) out.push(in.pop());out.pop();}}// Get the front element.public int peek() {if (!out.empty()) return out.peek();else {while(!in.empty()) out.push(in.pop());return out.peek();}}// Return whether the queue is empty.public boolean empty() {if(in.empty()&& out.empty()) return true;else return false;}
}
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